It has been the practice in the prior art to obtain highly accurate measurements of microwave or radio frequency power by employing bolometers and the D.C.-substitution technique. Self-balancing systems of this character are disclosed, for example, in Engen U.S. Pat. No. 2,997,652 and Larsen et al. U.S. Pat. No. 3,611,130. These prior art self-balancing systems make use of a Wheatstone bridge, one arm of which comprises a bolometer element, such as a thermistor or barretter. An amplifier which is connected across one diagonal of the bridge senses the magnitude and direction of unbalance of the bridge and forces a current through the other diagonal of the bridge so as to drive the bolometer resistance up or down until the bridge is balanced.
Although systems of this kind are highly accurate, they are nevertheless subject to a number of drawbacks. As R.F. power is dissipated in the bolometer, the common-mode voltage at the amplifier input changes. For this reason, an amplifier having a very high common-mode rejection ratio (CMRR) is required for accurate measurement of the substituted D.C. power.
Since the terminal surface of the R.F. portion of a system often dictates the location of the bolometer mount, the bolometer is typically located at some distance from the other three arms of the self-balancing bridge. It is not common practice, nor is it usually feasible, to separate the resistances associated with the leads to the bolometer from the resistance of the bolometer itself. This causes a first-order error in the measurement of the substituted power, unless the actual lead resistances are measured separately and corresponding corrections are calculated for each different set of leads.
The degradation of the signal-to-noise ratio is the most serious and most subtle problem associated with the prior art systems. Equivalent noise voltage generators may be considered as located in the series with each of the input leads of the balancing amplifier. If there is no feed-back from the output of the amplifier to the inverting input, the noise will be amplified by the full open-loop gain of the amplifier and will appear at the top of the bridge, the point from which the output signal is taken. Microvolts of noise at the input of the amplifier would thus result in volts of noise at the output, and this would be particularly true if the bridge were perfectly balanced at all frequencies. It is the purpose of the system to maintain the bridge in balance as exactly as possible, thus resulting in noise at the output at those frequencies where the attenuation of the feed-back is high and the amplifier has gain. In actuality, there is feed-back, but only to the extent that the amplifier fails to balance perfectly the bridge in a dynamic sense. These two conflicting requirements result in a serious degradation of the signal-to-noise ratio at the top of the bridge. This degradation is particularly serious when small R.F. power levels are to be measured, because the change in the D.C. voltage at the output is also then small. This conflict is an intrinsic defeat in all systems which use a self-balancing bridge technique.